Motor vehicle manufacturers are constantly striving to provide vehicles which, in the event of a collision, reduce the risk of injury to persons involved in the collision. These persons may be occupants of the vehicle or a pedestrian which is struck by the vehicle. To this end, vehicles are nowadays designed with so-called deformation zones which deform in a controlled manner to thereby absorb energy which arises during impact between the vehicle and an object. The amount of energy which arises in a collision is proportional to the square of the relative velocity between the vehicle and the object at impact. Obviously, the risk of injury to occupants of vehicles is increased at higher speeds. Due to the considerable amounts of energy which arise as a result of high speed collisions, the deformation zones must exhibit a certain degree of stiffness, or resistance to deformation, to function effectively at those high speeds.
Most collisions between vehicles and pedestrians occur in built-up areas in which the speed of the vehicles is relatively low. For example, most jurisdictions impose a speed limit in built-up areas of about 50 km/h. Due to the relatively light weight of most pedestrians, the amount of energy which arises in a low speed collision between a vehicle and a pedestrian is relatively low. This implies that the deformation zones of the vehicle are not caused to deform to any great extent and therefore a large amount of the energy is transmitted to the pedestrian, possibly resulting in injury.
In an attempt to reduce pedestrian injury, the prior art has suggested various ways of reducing the stiffness of a vehicle during collision with a pedestrian. For example, in U.S. Pat. No. 6,050,624 a bumper mounting structure is disclosed having a dual rate shock absorbing member which offers less resistance to deformation at low impact energy amounts and higher resistance to deformation at higher impact energy amounts. In the bumper arrangement according to DE-A-199 42 167, longitudinal displacement of a sliding member is restricted by a pin at higher speeds or if a sensor detects the proximity of an object larger than a pedestrian. At lower speeds, the pin is disengaged from the sliding member, thereby facilitating displacement of the sliding member to progressively absorb the impact energy. EP-A-0 983 909 describes a stiffener assembly for a bumper system of a motor vehicle, which assembly includes a stiffener operatively connected to the bumper system and movable between an up position and a down position based on the speed of the vehicle.
A disadvantage with components which are readily deformable at low impact speeds is that such components may need to be replaced as a result of a low speed collision in which the risk of personal injury is negligible. Such may be the case, for example, in a collision at parking speeds between a vehicle and, say, a lamppost. Furthermore, in order to keep production costs low, any impact absorbing or transmitting arrangement should be as simple as possible.